In a conventional WDM network, a path protection means against transmission line fault, etc. is usually provided. In FIG. 1, an example of network path protection in a conventional WDM system is shown. A terminal station 2 accommodating a first client (client 1) and another terminal station 3 accommodating a second client (client 2) are interconnected through duplicated optical transmission line fibers 1.
In terminal stations 2 and 3 in the system shown in FIG. 1-A, optical path cross-connect switches 6 are provided at the location nearer to the client, being connected to wavelength multiplexing/demultiplexing equipment 4 through transponders (transmitters and receivers) 5.
On the other hand, in terminal stations 2 and 3 in the system shown in FIG. 1-B, dispositions of transponders 5 and optical path cross-connect switches 6 are allocated oppositely to those applied in the system shown in FIG. 1-A, in which optical path cross-connect switches 6 are directly connected to wavelength multiplexing/demultiplexing equipment 4.
In FIG. 2, there is shown an enlarged chart of terminal station 2 shown in FIG. 1-A. (Terminal station 3 also has the identical configuration.) Terminal station 2 is connected to duplicated optical transmission line fibers 1-1 and 1-2 through duplicated multiplexing/demultiplexing equipment 4-1 and 4-2, respectively.
In FIG. 2, terminal station 2 includes a pair of transponders 5-1 and 5-2 and a pair of optical path cross-connect switches 6-1 and 6-2, respectively corresponding to the duplicated wavelength multiplexing/demultiplexing equipment 4-1 and 4-2.
Each transponder 5-1, 5-2 is provided with monitoring circuits 50 for monitoring signals transmitted through up/down links of wavelength multiplexing/demultiplexing equipment 4-1 and 4-2.
Optical path cross-connect switch 6-1 is connected to transmission line fibers of the duplicated transmission lines 1-1 and 1-2 directed to the first client. Optical path cross-connect switch 6-1 has a monitoring circuit 60 for monitoring respective signals transmitted through transmission lines 1-1 and 1-2. Similarly, optical path cross-connect switch 6-2 is connected to transmission line fibers of the duplicated optical transmission lines 1-1 and 1-2 directed to the second client.
In addition, in optical path cross-connect switch 6-2, there is also provided a monitoring circuit for monitoring signals transmitted through the transmission line fibers directed to the second client. This monitoring circuit is similar to monitoring circuit 60 provided in optical path cross-connect switch 6-1, and is not shown in FIG. 2.
In terminal station 2, outputs of monitoring circuits 50 of transponders 5-1 and 5-2 are input to a control circuit 21. Also outputs of monitoring circuits 60 of optical path cross-connect switches 6-1 and 6-2 are input to a control circuit 21.
In this control circuit 21, monitoring outputs from monitoring circuits 50 and monitoring circuits 60 are compared with control values input from a processor 7. Based on the result of the above comparison, control circuit 21 controls a selector 22 to switch over optical path cross-connect switches 6-1 and 6-2, in case of a fault, from a transmission line fiber having a fault to a standby transmission line fiber. Thus a means against fault is realized.
In FIG. 3, there is shown an enlarged drawing of terminal station 2 shown in FIG. 1-B, of which configuration is also applied to terminal station 3. Terminal station 2 is connected to the duplicated optical transmission line fibers 1-1 and 1-2 respectively through a duplicated sets of wavelength multiplexing/demultiplexing equipment 4-1 and 4-2.
In FIG. 3, terminal station 2 includes a pair of optical path cross-connect switches 6-1 and 6-2 each corresponding to the duplicated wavelength multiplexing/demultiplexing equipment 4-1 and 4-2.
Optical path cross-connect switch 6-1 is connected to the transmission line fibers of the duplicated optical transmission line fibers 1-1 and 1-2, being directed to the first client. Optical path cross-connect switch 6-1 has a monitoring circuit 60 for monitoring respective signals thereon. Similarly, optical path cross-connect switch 6-2 is connected to a transmission line fibers of the duplicated optical transmission line fibers 1-1 and 1-2 directed to the second client.
In addition, there is also provided a monitoring circuit for monitoring signals on the transmission line fiber directed to the second client. This monitoring circuit is similar to monitoring circuit 60 provided in optical path cross-connect switch 6-1, and is not shown in FIG. 3.
A line signal passed through the switched connection of optical path cross-connect switches 6-1 is input to a common transponder 5. In the opposite direction, a signal output from transponder 5 is input to optical path cross-connect switches 6-2. Transponder 5 includes monitoring circuits 50 for monitoring a signal transmitted on a transmission line fiber to the first clients, as well as a signal transmitted on a transmission line fiber in an opposite direction to the second clients.
In terminal station 2, outputs of monitoring circuits 50 of transponders 5 and monitoring circuits 60 of optical path cross-connect switches 6-1 and 6-2 are input to control circuit 21.
In this control circuit 21, monitored outputs from monitoring circuits 50 and monitoring circuit 60 are compared with control valued input from processor 7. Base on the result of the above comparison, control circuit 21 controls a selector 22 to switch over optical path cross-connect switches 6-1 and 6-2, from a transmission line fiber having a fault to a standby transmission line fiber.
In the system configurations shown in FIGS. 2 and 3, terminal stations 2 and 3 are so configured as to interconnect with the duplicated optical transmission line fibers 1-1 and 1-2. In such configurations, terminal stations 2 and 3 are interconnected with a one-to-one i.e. point-to-point connection.
In FIG. 4, there is shown another example of wavelength multiplexing transmission network, in which a plurality of nodes N1 to N4 are interconnected by transmitting signals successively from one node to the neighboring node. Namely, each connections between nodes N1 and N2, nodes N1 and N4, nodes N2 and N3, and nodes N3 and N4 is configured with a point-to-point connection, which is similar to the configuration shown in FIG. 1. Each node is provided with a function of terminating wavelength multiplexed signals having wavelengths λl to λn received from the other node.
In FIG. 4, node N1 and node N2 are interconnected via a repeater RP. Even in this case, each node N1/N2 is provided with the same terminating function as mentioned above.
It will be a problem in such a future WDM network that is constituted by a ring network or interconnection of such ring networks, because it is not possible to realize optical transmission line protection by the method shown in FIG. 1 for such networks having ring configuration.
Moreover, in such a system having duplicated optical transmission line fibers as shown in FIG. 1, two wavelengths (λ) must be allocated: one wavelength for a working transmission line and the other wavelength for standby transmission line. In other words one wave cannot be used at any time. This produces reduced transmission capacity against the transmission capacity logically induced.